1. Field of the Invention
The present invention relates to a method for fabricating a vertical CMOS image sensor, and more particularly, to a method for fabricating a vertical CMOS image sensor that reduces or minimizes the lateral diffusion in an (implant) plug by controlling the kind (or element), dose and energy of ion implantation.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device for converting an optical image to an electric signal. Image sensors may be classified into charge-coupled devices (CCDs) and CMOS image sensors. In the case of the CCD, metal-oxide-silicon MOS capacitors are positioned adjacent to one another, and electric carriers are stored in and transferred from the MOS capacitors. In the CMOS image sensor, the number of MOS transistors generally corresponds to the number of pixels. CMOS technology is generally used to form a control circuit and a signal processing circuit as peripheral circuits, whereby output signals are sequentially output using the MOS transistors.
In a vertical CMOS image sensor according to the related art, a plug having a depth of about 2 μm (which may be used, e.g., to sense a signal of a red photodiode) is typically formed by implanting phosphorous at a high energy and dose. At sufficiently high doses, there may be relatively severe lateral diffusion. Thus, the isolation characteristics between adjacent photodiodes may deteriorate.
Hereinafter, a method for fabricating a vertical CMOS image sensor according to the related art will be described as follows.
FIG. 1 is a cross sectional diagram showing a method for fabricating a vertical CMOS image sensor according to the related art.
As shown in FIG. 1, a red photodiode 11 is formed in a first epitaxial layer (not shown), and a second epitaxial layer 12 is grown thereon to a thickness of about 2 μm. Then, a first photoresist pattern (not shown) is formed on the second epitaxial layer 12, wherein the first photoresist pattern (not shown) has an open portion for a plug. For electrical connection with the red photodiode 11, phosphorous ions are implanted into the second epitaxial layer 12 at a high energy (e.g., of 1200 KeV) to form plug 13 and at a medium energy (e.g., of 500 KeV) with the first photoresist pattern as a mask. After removing the first photoresist pattern, a second photoresist pattern (not shown) is formed on the second epitaxial layer 12. Then, a green photodiode 14 is formed in the second epitaxial layer 12 by ion implantation, and the second photoresist pattern is removed.
Next, a third epitaxial layer 15 is formed on the second epitaxial layer 12 including the green photodiode 14, and an STI (Shallow Trench Isolation) layer 16 is formed in the third epitaxial layer 15. Then, a third photoresist pattern (not shown) is formed on the third epitaxial layer 15, and a second plug 17 is formed in the third epitaxial layer 15 by ion implantation.
Also, a fourth photoresist pattern (not shown) is formed on the third epitaxial layer 15 including the STI layer 16, and a blue photodiode 18 is formed by ion implantation.
In the aforementioned vertical CMOS image sensor according to the related art, the isolation characteristics between adjacent photodiodes may deteriorate due to the lateral diffusion of the first plug. Accordingly, it can be difficult to decrease the size of unit pixel due to the deterioration of the isolation characteristics.
The method for fabricating the vertical CMOS image sensor according to the related art has the following disadvantages.
Where an implant plug may be formed at a depth of about 2 μm (e.g., to sense the signal of the red photodiode 11), the lateral diffusion may be severe due to the phosphorous implant of high dose (and perhaps to some extent, high dose in combination with high energy). Thus, it may have a limitation with regard to decreasing the size of the unit pixel due to the potential deterioration of the isolation characteristics. As a result, it can be difficult to obtain (or realize the potential for) high integration of the vertical CMOS image sensor, and thus decrease the production cost.